The present invention relates generally to the formation of hydrogels, and, especially, to formation of hydrogels suitable to increase the strength (that is, wet and/or dry strength) of paper and to methods of increasing the strength of paper using such hydrogels.
For many years, the paper-making industry has sought ways of increasing the strength of paper. In that regard, papers fabricated without some additional means of reinforcement thereof can fall apart upon wetting or when subjected to mechanical stress in the dry state. Typically, a material is added to the wet pulp to improve the strength thereof during the formation of sheets prior to ultimate drying. Maintaining paper strength upon wetting is desirable in many applications, including bathroom tissue, paper towels, napkins, and the like. Moreover, additives which increase the strength of a wet paper often increase the dry strength of that paper. Increased dry strength is desirable, for example, in various packaging applications.
A number of wet- and dry-strength increasing additives are known in the art. However, such compositions typically include one or more components which are environmentally unfriendly or even toxic.
For example, some wet-strength additives are condensation products of urea and formaldehyde. Polyamine can be added to make such resins cationic. Other wet-strength additives include organochlorine crosslinked amidoamine compounds. A discussion of wet-strength additives and their mechanisms is presented in “The 65 Mechanism of Wet-Strength Development in Paper: A Review,” by Herbert H. ESPY, Tappi Journal, Vol. 78, No. 4, pages 90-97 (April 199 as well as in “Chemistry of Paper Wet-Strength. I. A Survey of Mechanisms of WetStrength Development,” by Lars WESTFELT, Cellulose Chemistry and Technology, Vol. 13, pages 813-825 (1979), the contents of which are incorporated by reference as though set forth in full herein.
Chemical compositions purported to increase paper wet strength while being chemically benign or environmentally friendly are set froth in U.S. Pat. No. 6,146,497, which describes a composition including (a) a water-soluble polymeric material comprising at least one nucleophilic polymer, (b) a phenolic compound (phenols or polyphenols) and (c) a component (an oxidizing agent) capable of converting the phenolic compound into a quinone compound. Sugars in conjunction with their corresponding oxidases are contemplated as potential oxidizing agents for the phenolic component of the composition of U.S. Pat. No. 6,146,497. Many phenolic compound, however, are environmentally undesirable. Moreover, many oxidizing agents are also environmentally undesirable (for example, potassium dichromate and potassium permanganate).
It thus remains desirable to develop improved, environmentally friendly compositions for increasing paper wet and/or dry strength.